1. Field of the Invention
The present invention relates to a bipolar transistor as well as to a method of producing the bipolar transistor, the transistor having an epitaxial base in a substrate.
2. Description of Prior Art
As the integration density of modern microelectronic circuits increases, it is necessary for the integrated devices, i.e. bipolar transistors, to have small dimensions as well as good electrical properties, so that the integrated circuits take up little space and operate with as little loss as possible. As information rates are on the increase, it is important, in addition, for the integrated devices to have a good high-frequency performance, which may be achieved, for example, by reducing the parasitic effects by reducing the extrinsic base resistance of a bipolar transistor or by reducing its base-collector capacitance.
For producing a conventional double-polysilicon bipolar transistor having an epitaxed base, a base layer (base) is deposited, usually in an epitaxially selective manner, on a semiconductor substrate, for example a silicon substrate. To deposit the base in an emitter window of a bipolar transistor and, at the same time, to obtain a self-adjusted connection to a highly conductive base-terminal polysilicon required for contacting the base, a sacrificial layer between the substrate and the base-terminal polysilicon in the emitter window is often undercut in etching, since the deposition of a base grown in a selectively epitaxial manner in a double-polysilicon bipolar process usually requires the presence of the sacrificial layer above the silicon substrate in an area of the emitter window. In order to use this double-polysilicon concept for epitaxed transistors, a selective deposition is further required which grows only on silicon surfaces. By undercutting the base-terminal polysilicon, the base may epitaxially grow on the substrate surface, the thus defined epitaxial base (e.g. silicon germanium, SiGe) also growing on the highly doped base-terminal polysilicon from below (in principle, other materials are also suitable, and, in particular, silicon (without germanium) has already been employed). This results in a self-adjusted base terminal. However, a problem here is that the (e.g.) SiGe which is growing on the polysilicon from below, is polycrystalline and will at some point butt against the epitaxial SiGe growing on the silicon substrate, whereby an interface of these two layers is formed, which interface may lead to problems regarding a reproducibility of a bipolar transistor as well as with regard to a stability of same. In addition, epitaxed bases are almost exclusively used as SiGe epitaxy. Since the SiGe base grows on the terminal polysilicon in a polycrystalline manner and grows on the silicon substrate in a crystalline manner, the interface of the SiGe polysilicon with the epitaxial SiGe will lead to further problems, such as a poor reproducibility of the electrical parameters of a bipolar transistor thus produced, and thus, for example, to poor matching properties. In principle, however, other materials are also suitable, and, in particular, silicon (without germanium) has already been employed.
The presence of the relatively thick sacrificial layer (70–150 nm) further leads to a very high emitter stack which is processed-induced. In particular in modern BiCMOS technologies, the high emitter stack leads to planarization problems before a via hole is etched, which, in addition, sets process-induced limits to a further reduction of dimensions of such a bipolar transistor. A further disadvantage of such an approach for producing a bipolar transistor is the fact that the formation of the sacrificial layer required leads to the manufacturing process becoming more expensive.
The dimensions of a bipolar transistor may be reduced if the sacrificial layer is not formed in a manufacturing process. In WO01/63644 A2, an emitter via hole is etched through a highly doped implanted substrate layer. A base layer is produced in the emitter via hole. However, the base-collector capacitance produced with this method is high, which leads to a deterioration of the high-frequency performance of the bipolar transistor. Another disadvantage in the bipolar transistor disclosed in the above-mentioned document is that, in the manufacturing process, a platform collector needs to be produced by means of an implantation, which is why the manufacturing costs go up.